1. Field of the Invention
This invention relates to a method for producing a dense silicon nitride (Si.sub.3 N.sub.4) compact with one or more crystallized intergranular phases. More particularly, this invention relates to a process for producing a dense silicon nitride compact using as a sintering aid preformed glass in particulate form and composition capable of crystallization upon subsequent heating after densification of the silicon nitride.
2. Description of the Related Art
The use of nitrogen ceramics as high temperature structural components has increased dramatically in the past few years. The bulk of this increased usage and the correspondingly increased research has revolved around the use of silicon nitride (Si.sub.3 N.sub.4) in advanced heat engine components. The use of silicon nitride for these applications stems from its excellent high temperature properties including very good thermal shock resistance due to a low linear coefficient of thermal expansion, as well as high strength and refractory properties.
The outstanding properties of silicon nitride (Si.sub.3 N.sub.4) single crystals at high temperatures reflect a strong degree of covalent bonding which results in rather formidable problems in conventional ceramic powder processing techniques. Bulk and boundary diffusion coefficients for silicon nitride are orders of magnitude lower than for refractory oxides in the same temperature regime.
Thus, neck growth proceeds by the non-densifying processes of surface diffusion and evaporation/condensation resulting in a low strength ceramic due to the large pore fraction retained.
Processing techniques involving hot-pressing or hot isostatic pressing with the addition of oxide densification aids have resulted in the attainment of nearly theoretically dense pieces of silicon nitride. Such sintering aids promote densification through the formation of a low melting point eutectic resulting from the reaction of the densification aid and the surface silica layer present on the silicon nitride powder. However, the resulting ceramic has an undesirable microstructure which contains a glassy phase at grain boundaries and multi-grain junctions. This glassy phase is thought to be responsible for the loss of strength at high temperatures due to a grain boundary sliding mechanism which becomes active when the softening point of the glass is reached.
An obvious alternative, short of eliminating the undesirable intergranular phase entirely, is to increase its softening point, i.e., its refractoriness, into a temperature regime above the temperature range at which the ceramic will see service. This can be accomplished through the use of rareearth oxides as the densification additive. Of the rare-earth oxides, Y.sub.2 O.sub.3 has the highest melting point (2410.degree. C.) which suggests that formation of an intergrannular phase containing it will have correspondingly high softening, glass transition, and melting temperatures. Al.sub.2 O.sub.3 additions to sintering additive compositions containing rare-earth oxides have been found to promote densification while suppressing the catastrophic oxidation experienced by Y.sub.2 O.sub.3 -fluxed silicon nitride at intermediate temperatures of 600.degree.-1000.degree. C.
For these reasons Y.sub.2 O.sub.3 /Al.sub.2 O.sub.3 -fluxed silicon nitride (Si.sub.3 N.sub.4) has met with increasing technological success. Thus, Fukuhara et al U.S. Pat. No. 4,609,633 discloses a silicon nitride sintered compact formed using sintering aids or auxiliaries which comprise at least one oxide, nitride, or oxynitride of rare earth elements consisting of Sc, Y, and lanthanide elements; and at least one oxide, nitride, or oxynitride of Group IIA elements, and/or Group IIA elements, e.g., Al.sub.2 O.sub.3. Powders of these materials are mixed with powders of silicon nitride prior to sintering to form the desired silicon nitride compact.
Hsieh U.S. Pat. No. 4,552,851 describes a process for forming a yttrium aluminate sintering aid for silicon nitride bodies which comprises prereacting oxides of yttrium and aluminum to form yttrium aluminate by heating a mixture of the oxides to 1000.degree. to 1450.degree. C. for a period of 3 to 20 hours. The pre-formed yttrium aluminate is then mixed with silicon nitride and the mixture is consolidated preferbly by hot pressing or hot isostatic pressing to form a dense silicon nitride compact.
Compositions containing such rare-earth oxides and aluminum oxide have even been used as bonding materials to bond or braze together previously formed silicon nitride parts. For example, Layden U.S. Pat. No. 4,384,909 discloses bonding or brazing compositions for bonding together previously formed ceramic parts of silicon nitride which brazing compositions may comprise Si.sub.3 N.sub.4, Al.sub.2 O.sub.3, and Y.sub.2 O.sub.3 and either AlN or SiO.sub.2. The composition which included SiO.sub.2 was said to be a highly viscous fluid which cooled to a glass while the composition containing AlN was said to be expected to crystallize upon cooling. In using the compositions for bonding parts made from silicon nitride, the materials were blended with methanol to a smooth creamy consistency and the methanol slurry was placed between the parts to be joined. The assemblies were then fired at 1600.degree. C. and then cooled.
It has also been recognized that conversion of the glassy intergranular phase of a sintered silicon nitride compact into a crystalline phase will improve the high temperature mechanical properties of such silicon nitride structures.
Ezis U.S. Pat. No. 4,264,548 discloses a method of making a silicon nitride object by forming a mixture of powders of silicon nitride (Si.sub.3 N.sub.4) containing SiO.sub.2 as an oxide coating, Y.sub.2 O.sub.3, and Al.sub.2 O.sub.3. After hot pressing at 1680.degree.-1750.degree. C., the object is relieved of pressure and temperature and allowed to cool. It is then heat treated at 1000.degree.-1400.degree. C. for a time period sufficient to provide a nucleating reaction in secondary phases formed as a result of the hot pressing. The resulting object is said to contain one or more of three crystallized forms consisting of 5Y.sub.2 O.sub.3.4SiO.sub.2 Si.sub.3 N.sub.4 ; 2Y.sub.2 O.sub.3.SiO.sub.2 Si.sub.3 N.sub.4 ; and Y.sub.2 O.sub.3.SiO.sub.2.
Smith et al U.S. Pat. No. 4,280,850 also describes a polycrystalline silicon nitride having a substantially crystalline intergranular phase formed by ball milling a densification aid such as Y.sub.2 O.sub.3 with silicon nitride powder. The powder mixture may then be hot pressed at 1675.degree.-1800.degree. C. at 3000-5000 psi to form the dense compact. Crystallinity of the intergranular phase by exclusion of amorphous or glassy material is said to be achieved by either controlling the composition during processing to insure exclusion of glass stabilizing additives or impurities or by giving the compact a post-sintering crystallization heat treatment, e.g., heat treated for 5 hours at 1525.degree. C.
Buljan U.S. Pat. No. 4,179,301 describes the production of a silicon nitride having a crystalline intergranular phase formed by adding to a powder mixture of silicon nitride and a densifying agent (such as an oxide of yttrium or magnesium) a nucleating agent of elemental Ti or Fe or as an oxide or nitride of these elements. The powder mixture is formed into a compact which is sintered at 1700.degree. C. The compact is then cooled at 500.degree. C. per hour. The patentee shows that the addition of the nucleating agent results in improved high temperature strength.
While it is, therefore, recognized that a densified silicon nitride compact containing crystalline intergranular phases will have superior high temperature properties, formation of such crystalline forms has always been carried out by heating of powdered mixtures of various densification agents and silicon nitride powder to sinter the silicon nitride by forming glassy intergrannular phases between the silicon nitride particles followed, in such cases, by subsequent formation of the crystalline phases either during controlled cooling of the sintered object or during subsequent heating steps. If the various powders comprising the densification materials and silicon nitride are not thoroughly mixed together, a nonuniform, i.e., nonhomogeneous glass may be formed comprising areas of different phases or areas which remain amorphous during subsequent attempts at crystallization. Furthermore, if the ratios of the densification materials used do not form crystalline phases, the desired subsequent formation of crystalline intergrannular phases can not be achieved.